Is negative air pressure in a room bad?

Negative air pressure in a room is not inherently bad, it depends on whether it is intentional and controlled or unintentional and excessive. In hospitals, negative air pressure is intentionally created in isolation rooms to prevent airborne diseases like tuberculosis from escaping, which is good and life-saving. In that context, the room is designed to have lower pressure than the hallway so air flows inward and is exhausted through HEPA filters. However, in a regular home, unintentional negative air pressure can cause problems. If your house has negative pressure because exhaust fans, dryers, fireplaces, or HVAC systems pull more air out than comes in, it can cause backdrafting where combustion appliances like furnaces and water heaters pull dangerous carbon monoxide back into the house instead of venting outside, which is very dangerous. It can also cause cold drafts, increased energy bills because outside air is sucked in through cracks, doors that are hard to open, and moisture problems where humid outside air is pulled into walls causing mold. Mild negative pressure is normal in tightly sealed homes, but strong negative pressure indicated by whistling doors, lingering cooking smells that won't exhaust, or a fireplace that smokes into the room suggests a problem that should be fixed by balancing ventilation, adding makeup air, or adjusting HVAC fans.